DESCRIPTION (From the applicant's abstract): In contrast to classical neurotransmitters that are discretely released at synapses, adenosine is produced and released by all cells and diffuses into interstitial fluid. Adenosine exerts potent biological effects by activating specific receptors. The receptors include A1 adenosine receptors (A1ARs), which couple with Gi and Go and inhibit cAMP accumulation within cells. In the mammalian brain, A1ARs are widely expressed, with highest concentration found in the hippocampus. We have discovered that a unique feature of A1Ars is their high expression on axons. We have also found that A1ARs are expressed in the brain during critical periods of development. We also find that A1Ar activation in the neonatal period leads to significant reductions in total axon volume, indicating that A1AR impairs axon formation. Showing that adenosine can act directly on neurons to inhibit axon growth, A1AR agonists slow axonal growth in cultured hippocampal neurons. Although it is recognized that G proteins are heavily expressed on axons, the receptor systems that regulate axonal G proteins are largely unknown. We hypothesize that adenosine is an important regulator of axonal G proteins and that activation of axonal A1ARs during development inhibits axon growth. We also hypothesize that activation of A1ARs during the neonatal period may impair brain development. This application presents a focused series of studies aimed at testing this hypothesis and elucidating the mechanisms of neuronal A1AR action. To test our hypothesis (1) we will examine the effects of adenosine on axon formation in vivo. (2) We will examine the effects of A1Ar activation on axon growth in cultured hippocampal neurons. (3) We will elucidate the mechanisms by which A1AR activation inhibits axon growth. We anticipate these studies will lead to new insights into the mechanisms of A1AR action. We also anticipate that these studies will identify A1ARs as an important regulator of axon growth.